One type of imaging system uses photoconductive material to absorb incident radiation and to form and hold a latent image of an object, in the form of a distribution of charge carriers. Readout of the latent image and conversion of it to electronic form is achieved by scanning a narrow beam of radiation across the photoconductive material, and detecting the motion of charges caused thereby. The charge movements for each scanned spot (which represent the pixels of the scanned image) are received by an electrode, integrated and digitized in an appropriate form to be used to create a digital representation of the latent image. An example of this type of system is disclosed in U.S. Pat. No. 4,176,275 (Korn et al.).
Because of the amount of time involved in charge migration and collection for each spot illuminated by the scanning beam, the time period for scanning and processing of images from large photoconductive structures can be considerable. This is especially true when the overall system resolution is very high, creating a very large number of pixels. The system disclosed in U.S. Pat. No. 5,268,569 (Nelson et al.) provides improvements in scanning speed by using a plurality of elongated parallel strips as the readout electrodes, with the strips connected respectively to a plurality of charge amplifiers. Using this system, a scan line can be spread across the electrodes in a space and time pattern in such a manner as to allow for charge migration and detection time at each strip electrode-amplifier, before the scanning beam returns to the electrode. Although this system does provide improvements in performance, certain problems can be encountered in the practical application of these principles to high performance, high resolution imaging systems.
Because of transverse migration or diffusion of charges within the photoconductive material, it cannot be assumed that all of the charge movement caused by a particular readout scan spot will be collected by the electrode strip immediately underneath the spot. This is especially true if the spot in question is close to a boundary between adjacent electrode strips. It is therefore necessary to collect charge movements from the strip electrode beneath the spot, and also from any electrode close enough to receive some of the charge, and to combine the charges collected by such electrodes, in order to get an accurate representation of the value of the scanned spot, or pixel. In other words, a scanned spot on the photoconductive surface, which is to correspond to a defined pixel of the latent image being scanned, does not provide a one-to-one correspondence with charge received by the electrode strip beneath the spot. It may be necessary to receive charge at one or more adjacent electrodes also, and to combine them in order to obtain the correct value for the pixel corresponding to the scanned spot.
Another problem in the practical application of such systems can arise from errors in the positioning of the photoconductive structure relative to the scanning-readout apparatus. In one type of system, the photoconductive structures which have previously been exposed with radiation to form the image thereon, are placed in some type of holder or cassette in a scanning structure, for readout and image digitization. Due to small tolerances in various parts of the system, a photoconductive structure, and in particular its plurality of elongated strip electrodes, may not be positioned exactly in the nominal or intended position with respect to the scanning apparatus. This may result in the pattern of scan spots being displaced from their expected positions with respect to the strip electrodes in the photoconductive structure. Such misalignment, for example, could result in a given pixel being scanned on the photoconductive structure atop the neighboring electrode strip, rather than the expected or nominal one. Also, such positioning errors could result in the displacement of a given scan pixel such that, while it still overlies the nonfinal or intended electrode strip, its position is shifted so that the identity of the closest adjacent strip switches sides. These effects must be compensated for if the proper electrodes are to be used in integrating and combining charges to obtain the correct pixel value for a scanned spot.
While tolerances can be held to close values through careful design and manufacture of the photoconductive structure, cassette and scanning apparatus, it is not practical to eliminate them completely, because of the very tight tolerances involved, especially in the case of very high resolution systems with correspondingly small pixel size and narrow electrode strips. It is therefore important to provide a way to measure and compensate for the expected errors.